To study protein localization signals you have engineered yeast strains that exp

Question

To study protein localization signals you have engineered yeast strains that express the indicated fusion proteins below.

A) A PMP fusion protein with an amino terminal hydrophobic signal sequence and a positively charged carboxy terminal signal PMP (Protein Misfolding Peptide) is toxic to yeast only if it accumulates in the ER.

Predict if expression of the PMP fusion protein will impact yeast viability. Be sure to justify your answer in the context of protein localization?

B) Cathepsain X with an amino terminal amphiphilic alpha helix sequence and a carboxy terminal KDEL.

When present in growth media, chaotropic salts disrupt protein folding and impair yeast growth. If cathepsain X is localized to the ER lumen, it facilitates protein folding and allows yeast to grow in the presence of these salts. Predict how expression of cathepsain X fusion protein will influence yeast growth when cells are incubated in media containing chaotropic salts. Be sure to justify your answer in the context of protein localization.

Explanation / Answer

A) Peroxisomal Membrane Proteins (PMP) belong to the basic repertorie of organelles and have amino terminal hydrophobic signalling sequences, so they can localize the fusion protein in the cell membrane. Pex proteins are homologous to proteins of the ER associated with degradation pathway. So, there chance of having impact on yeast viability. If these proteins are not accumulated, there will be no impact on growth or viability of yeast.

B) Cathepsin X is an enzyme that catalyses the release of C-terminal amino acid residues with broad specificty. But it lacks the action of C-terminal proline. Since many of the soluble ER proteins in mammalian cells bear a carboxy terminal tetrapeptide sequence, Lys-Asp-Glu-Leu (KDEL), which has been demonstrated to be both necessary and sufficient for the retention of at least one of these proteins (BiP) in the ER. The growing list of luminal ER proteins that bear this sequence implicated as the distinctive feature that signals retention in the ER. It is clear that the KDEL sequence functions as an ER retention signal.

Since, cathepsain X is localized to the ER lumen, due to the presence of KDEL, it facilitates protein folding and allows yeast to grow in the presence of these salts.

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